Superlattice Structure

ABSTRACT

A superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers is provided. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation-in-part of U.S. patent application Ser. No. 14/675,596, which was filed on 31 Mar. 2015, which is hereby incorporated by reference. U.S. patent application Ser. No. 14/675,596 is a continuation-in-part of U.S. patent application Ser. No. 13/803,718, which was filed on 14 Mar. 2013, and which claims the benefit of U.S. Provisional Application No. 61/610,636, which was filed on 14 Mar. 2012 and U.S. Provisional Application No. 61/768,799, which was filed on 25 Feb. 2013, all of which are hereby incorporated by reference. Additionally, U.S. patent application Ser. No. 13/803,718 is a continuation-in-part of U.S. patent application Ser. No. 13/162,895, which was filed on 17 Jun. 2011, which is a continuation-in-part of U.S. patent application Ser. No. 12/987,102, which was filed on 8 Jan. 2011, and which claims the benefit of U.S. Provisional Application No. 61/293,614, which was filed on 8 Jan. 2010, all of which are hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of the Grant No. IIP-0839492 awarded by the National Science Foundation.

TECHNICAL FIELD

The disclosure relates generally to semiconductor devices, and more particularly, to a superlattice structure configured to reduce polarization effects of the semiconductor materials forming the devices.

BACKGROUND ART

In nitride based semiconductor materials and devices, including visible and ultraviolet (UV) light emitting diodes (LEDs), polarization effects play a dominant role causing strong built-in fields and spatial separation of electrons and holes. These polarization effects can negatively impact the performance of nitride-based visible and ultraviolet light emitting diodes. For example, FIGS. 1A-1C show illustrative band diagrams of a positive-intrinsic-negative (p-i-n) quantum well structure according to the prior art. In particular, FIG. 1A shows a band diagram of the structure without external bias and illumination; FIG. 1B shows a band diagram of the structure with the p-i-n field compensated by external bias; and FIG. 1C shows a band diagram of the structure with the total electric field compensated by external bias and intense optical excitation.

Polarization effects were evaluated for illustrative aluminum indium gallium nitride-based (Al_(x)In_(y)Ga_(1-x-y)N-based) multiple quantum well (MQW) structures. The MQW structures comprise an Al molar fraction in the quantum wells and barrier layers close to 20% and 40%, respectively, and In content in both the quantum wells and barriers of approximately 2% and 1%, respectively. The MQW structures comprise a total of three wells, each of which is two to four nanometers thick, separated by four five nanometer thick barriers.

Calculations indicated that the barriers and wells undergo tensions of 0.815% and 0.314%, respectively. These tensions correspond to piezoelectric charges at interfaces induced by this mismatch of −0.0484 coulombs per meter squared (C/m²) for the well and −0.0134 C/m² for the barrier. The polarization charge was calculated as −0.041 C/m² and −0.049 C/m² for the wells and barriers, respectively. The total electric field in the well for an alternating sequence of barriers and wells was found to be 1.2 Megavolts per centimeter (MV/cm). About fifty percent of the field was due to piezoelectric effect and the remaining fifty percent was caused by spontaneous polarization, both having the same direction. This corresponds to a 0.12 eV band bending in a one nanometer wide quantum well. Such band bending precludes using wide quantum wells in deep UV LEDs, which decreases the overall LED efficiency by limiting the MQW design optimization to very narrow (i.e., one to two nanometer thick) quantum wells.

SUMMARY OF THE INVENTION

Aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation.

A first aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second composition distinct from the first composition and a second polarization opposite the first polarization.

A second aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first composition having a first polarization for a first sub-layer of each of the plurality of periods; and selecting a second composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second composition is distinct from the first composition and the second polarization is opposite the first polarization.

A third aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization.

A fourth aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride composition and a first polarization, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second polarization opposite the first polarization.

A fifth aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first group III nitride composition having a first polarization for a first sub-layer of each of the plurality of periods, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and selecting a second group III nitride composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second group III nitride composition is distinct from the first group III nitride composition and the second polarization is opposite the first polarization.

A sixth aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization, wherein the first group III nitride-based composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization.

The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various aspects of the invention.

FIGS. 1A-1C show illustrative band diagrams of a p-i-n quantum well structure according to the prior art.

FIGS. 2A and 2B show illustrative structures according to the prior art and an embodiment, respectively.

FIG. 3 shows a conduction band diagram comparing a conduction band profile for a conventional quantum well and a conduction band profile for a quantum well according to an embodiment.

FIG. 4 shows another illustrative structure according to an embodiment.

FIG. 5 shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment.

FIGS. 6A and 6B show illustrative light emitting device structures according to embodiments.

FIGS. 7A and 7B show side and facing views, respectively, of an illustrative structure according to an embodiment.

FIGS. 8A and 8B show band variations of an illustrative structure according to embodiments.

FIG. 9 shows band variation and doping of another illustrative structure according to an embodiment.

FIG. 10 shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al_(x)Ga_(1-x)N alloy according to an embodiment.

FIG. 11 shows an illustrative chart for selecting an aluminum content of an AlGaN alloy to maintain a target transparency for a corresponding emitted wavelength according to an embodiment.

FIG. 12 shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment.

FIG. 13 shows possible molar fractions of indium and aluminum in an AlInGaN layer to achieve zero total polarization for the AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment.

FIG. 14 shows contour lines of polarization for various combinations of molar fractions of indium and aluminum in an AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment.

FIG. 15 shows an illustrative flow diagram for fabricating a circuit according to an embodiment.

It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation. As used herein, unless otherwise noted, the term “set” means one or more (i.e., at least one) and the phrase “any solution” means any now known or later developed solution.

Turning to the drawings, FIGS. 2A and 2B show illustrative structures 2, 10 according to the prior art and an embodiment, respectively. As illustrated in FIG. 2A, structure 2 includes a superlattice layer 4, which includes a plurality of repeating sub-layers 6A-6C. Each sub-layer 6A-6C can be separated from another sub-layer by a second set of sub-layers 8A-8B in the superlattice layer 4. Superlattice structure 4 can be configured to perform any type of function as part of a device incorporating structure 2. For example, sub-layers 6A-6C can comprise a set of quantum wells and sub-layers 8A-8B can comprise a set of barriers. In this case, superlattice layer 4 can comprise a multiple quantum well structure.

As shown in FIG. 2B, an embodiment of the invention provides a structure 10 including a superlattice layer 12 that is configured, for example, to reduce polarization effects. In particular, the superlattice layer 12 includes multiple periods 14A-14C, each of which includes two or more sub-layers 16, 18 having different compositions. Adjacent sub-layers 16, 18 in each period 14A-14C are configured to have polarizations (e.g., built-in electric fields) that at least partially cancel one another. For example, sub-layer 16 can comprise a spontaneous polarization having an opposite sign as a spontaneous polarization of sub-layer 18. Similarly, sub-layer 16 can comprise a strain-induced polarization having an opposite sign of a strain-induced polarization of sub-layer 18. Still further, one type of polarization in sub-layer 16 can have an opposite sign of another type of polarization in sub-layer 18, thereby reducing the net polarization present due to a combination of multiple types of polarizations (e.g., spontaneous and strain-induced).

In an embodiment, sub-layer 16 can comprise a positive or negative spontaneous polarization, while sub-layer 18 comprises the other of the positive or negative spontaneous polarization. In a more particular embodiment, the absolute values of the spontaneous polarizations of sub-layers 16, 18 are substantially equal, so that the net spontaneous polarization for the period 14A-14C is close to zero. In another embodiment, sub-layer 16 can comprise a strain-induced (e.g., piezoelectric) polarization due to stretching or compression, while sub-layer 18 comprises a strain-induced polarization due to the other of stretching or compression. In a more particular embodiment, the absolute values of the strain-induced polarizations of sub-layers 16, 18 are substantially equal, so that the net strain-induced polarization for the period 14A-14C is close to zero. It is understood that the respective spontaneous and/or strain-induced polarizations of sub-layers 16, 18 can be configured to only partially reduce the net spontaneous and/or strain-induced polarization for the period 14A-14C.

In still another embodiment, the spontaneous and/or strain-induced polarization of one sub-layer 16, 18 is configured to at least partially compensate the other of the spontaneous and/or strain-induced polarization of the other sub-layer 16, 18. For example, sub-layer 16 can comprise a spontaneous polarization of a first sign, and sub-layer 18 can comprise a strain-induced polarization of the opposite sign. In this case, the net polarization for the period 14A-14C will be reduced due to the two types of polarizations of the sub-layers 16, 18 compensating one another.

The various periods 14A-14C in superlattice layer 12 can be separated from one another by a set of additional sub-layers 20A-20B. In an embodiment sub-layers 20A-20B comprise inactive layers having no polarization. In another embodiment, each period 14A-14C comprises a quantum well, while each sub-layer 20A-20B comprises a barrier. In this case, superlattice layer 12 comprises a multiple quantum well structure. The periods (e.g., quantum wells) 14A-14C in superlattice layer 12 can be wider than the conventional sub-layers (e.g., quantum wells) 6A-6C. For example, in an embodiment, the width of superlattice layer 12 can be greater than two nanometers. In a more particular embodiment, the width of superlattice layer 12 is between approximately three nanometers and eight nanometers. In particular, periods 14A-14C will comprise a much smaller polarization field than that of a conventional sub-layer 6A-6C of a similar width. As a result, the detrimental confined Stark effect is avoided, which separates electrons and holes within a quantum well and prevents efficient radiative recombination.

FIG. 3 shows a conduction band diagram comparing a conduction band profile 22 for a conventional quantum well 6A (FIG. 2A) and a conduction band profile 24 for a quantum well 14A (FIG. 2B) according to an embodiment. As illustrated, the conduction band profile 24 comprises a more shallow profile than that of the conduction band profile 22. As a result, electrons in quantum well 14A can spread out within the quantum well 14A more than the electrons in quantum well 6A, providing for a more efficient radiative recombination.

Returning to FIG. 2B, superlattice 12 can perform any function as part of a device formed using structure 10. To this extent, superlattice 12 is located between a first layer 26 and a second layer 28 of the structure 10. In an illustrative embodiment, first layer 26 and second layer 28 can be formed from two dissimilar materials (e.g., two dissimilar nitride based semiconductor materials), and superlattice 12 can be graded in such a manner that it compensates (e.g., reduces) strain exerted by the dissimilar materials of layers 26, 28. For example, the lattice structure of each sub-layer 16, 18 of superlattice 12 can gradually change from a lattice structure similar to first layer 26 to a lattice structure similar to second layer 28.

While periods 14A-14C are each shown including two sub-layers 16, 18. It is understood that each period 14A-14C can include any number of sub-layers 16, 18. Similarly, while superlattice layer 12 is shown including three periods 14A-14C, it is understood that superlattice layer 12 can include any number of two or more periods 14A-14C. For example, FIG. 4 shows another illustrative structure 30 according to an embodiment. Structure 30 includes a superlattice layer 32, which comprises four periods 34A-34D that are separated by three sub-layers 36A-36C. Each period 34A-34D is formed by a set of six sub-layers of alternating compositions and polarizations. To this extent, each sub-layer of each period 34A-34D is immediately adjacent to one or two sub-layers having a different composition and an opposite polarization (e.g., spontaneous and/or strain-induced as described herein). In this manner, the periods 34A-34D can be made even wider than the conventional sub-layers 6A-6C of the prior art with smaller polarization fields than a conventional sub-layer 6A-6C of a similar thickness.

In an embodiment, structures 10 (FIG. 2B) and 30 (FIG. 4) can comprise nitride-based heterostructures. In a more specific embodiment, the structures 10, 30 comprise group III nitride-based heterostructures. In this case, the periods 14A-14C, 34A-34D of each structure 10, 30, respectively, each can be formed of group III nitride materials. Group III nitride materials comprise one or more group III elements (e.g., boron (B), aluminum (Al), gallium (Ga), and indium (In)) and nitrogen (N), such that B_(W)Al_(X)Ga_(Y)In_(Z)N, where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitride materials include AlN, GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBN with any molar fraction of group III elements. In an even more specific embodiment, the sub-layers described herein are quaternary or ternary group III nitride sub-layers, such as such as AlInN, AlGaN, InGaN, or AlInGaN. For further strain and/or polarization reduction, one or more sub-layers 16, 18 forming each period 14A-14C, 34A-34D can be doped. The sub-layers 16, 18 can be doped p-type or n-type.

Furthermore, a sub-layer 16, 18 can comprise a monolayer.

FIG. 5 shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment. As illustrated, the calculated electric field drops to zero and goes negative as the indium molar fraction exceeds 0.7. In an illustrative embodiment, each sub-layer 16, 18 comprises AlInN with differing molar fractions of In. For example, sub-layer 16 can comprise an In molar fraction of approximately 0.65, which results in a calculated electric field of approximately 0.5 MV/cm, and sub-layer 18 can comprise an In molar fraction of approximately 0.77, which results in a calculated electric field of approximately −0.5 MV/cm. In this manner, the electric fields of both sub-layers 16, 18 can substantially cancel one another.

The superlattices 12, 32 described herein can be implemented as part of structures 10, 30 utilized for various types of devices, e.g., which are fabricated using semiconductor materials where polarization effects play a role. A superlattice 12, 32 described herein can be utilized as, for example, a multiple quantum well, an integral part of an ohmic and/or Schottky contact, a cladding layer, a buffer layer, a barrier layer, and/or the like, for the device. In an illustrative embodiment, structure 10 comprises a p-type contact including superlattice 12 and a metal layer 26 located thereon.

A structure 10, 30 described herein can be implemented as part of, for example, a light emitting device, such as a light emitting diode (LED), a superluminescent diode, or a laser. The light emitting device can comprise a visible light emitting device, an ultraviolet light emitting device, and/or the like. In this case, the light emitting device can include one or more superlattices as cladding layer(s), ohmic contact(s), and/or the like. In a more particular embodiment, the superlattice is formed as part of an ohmic contact for an ultraviolet light emitting device where a top p-type contact layer (e.g., layer 26 of FIG. 2B) of the ohmic contact, which is transparent to ultraviolet radiation, is located directly on the superlattice 12, 32. In a still more particular embodiment, the top p-type contact layer comprises AlInN.

It is understood that any combination of one or more layers (or sub-layers) in a structure 10, 30 can be configured to be at least partially transparent (e.g., semi-transparent or transparent) to radiation, such as ultraviolet radiation. As used herein, a layer is at least partially transparent to ultraviolet radiation if it allows more than approximately 0.001 percent of the ultraviolet radiation to pass there through. In a more particular embodiment, an at least partially transparent layer is configured to allow more than approximately five percent of the ultraviolet radiation to pass there through. In an embodiment, the at least partially transparent layer(s) are configured to be at least partially transparent to ultraviolet radiation emitted by the structure 10, 30. For example, the at least partially transparent layer(s) can be configured to be at least partially transparent to ultraviolet radiation in a range including the peak emission wavelength of the structure 10, 30 and at least five nanometers above and/or below the peak emission wavelength.

The layer(s) at least partially transparent to the ultraviolet radiation can be formed using any solution. For example, a transparent layer can comprise a p-type layer formed of a group III nitride material described herein. Illustrative at least partially transparent group-Ill nitride materials include AlGaN, AlInGaN, boron-containing alloys (GaBN, AlBN, AlGaBN, AlInGaBN, InGaBN, and/or the like), and/or the like.

Furthermore, the at least partial transparency of a layer can be achieved using any solution. For example, at least partial transparency can be achieved in materials with bandgaps smaller than a photon energy of the ultraviolet radiation due to tunneling, thermionic transport via impurity states, and/or the like.

Similarly, it is understood that any combination of one or more layers in a structure 10, 30 can be configured to reflect ultraviolet radiation. As used herein, a layer is reflective of ultraviolet radiation when it reflects more than approximately five percent of the ultraviolet radiation. In an embodiment, the reflective layer(s) are configured to reflect ultraviolet radiation emitted by the structure 10, 30. For example, the reflective layer(s) can be configured to reflect ultraviolet radiation in a range including the peak emission wavelength of the structure 10, 30 and at least five nanometers above and/or below the peak emission wavelength.

The ultraviolet reflective layer(s) can be formed using any solution. For example, a reflective layer can comprise a metal coating formed of Al, Rhodium (Rh), enhanced Al, enhanced Rh, Gold (Au), Aluminum Silicon Monoxide (AlSiO), Aluminum Magnesium Fluoride (AlMgF₂), and/or the like. Furthermore, the reflectivity of a layer can be achieved using any solution. For example, reflectivity can be achieved by the formation of a reflecting photonic crystal, a distributed Bragg reflector (DBR) structure, and/or the like.

The at least partially ultraviolet transparent and/or reflective layer(s) can comprise any of various layers of a structure 10, 30 based on a desired operating configuration for the structure 10, 30. For example, a structure 10, 30 can include an at least partially ultraviolet transparent contact. Such a contact can comprise, for example, a p-type at least partially ultraviolet transparent layer 26 (FIG. 2B) and an interlayer, such as an at least partially ultraviolet transparent superlattice 12 (FIG. 2B), for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like. Similarly, a structure 10, 30 can include an ultraviolet reflecting contact, which is configured to reflect a desired amount of the ultraviolet radiation generated by the structure 10, 30. Such a reflecting contact also can include, for example, a p-type ultraviolet reflective layer 26 and a superlattice 12 for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like.

A structure 10, 30 can include various other layers, which are at least partially ultraviolet transparent and/or ultraviolet reflective, such as a p-type superlattice 12, 32, an electron blocking layer located between a superlattice 12, 32 and a multiple quantum well structure, and/or the like. In each case, the at least partially ultraviolet transparent and/or ultraviolet reflective layer can be formed using any type of material. In an embodiment the at least partially ultraviolet transparent and/or ultraviolet reflective layer is formed using a group-Ill nitride material, such as boron-containing layers.

FIGS. 6A and 6B show illustrative light emitting device structures 40A, 40B according to embodiments. As illustrated in FIG. 6A, the device structure 40A comprises an n-type contact layer 50 adjacent to a radiation generating structure 52. Radiation generating structure 52 can comprise any type of structure, such as a multiple quantum well structure, for generating any type of radiation, such as ultraviolet light. Furthermore, device structure 40A includes a p-type contact layer 54 on an opposing side of the radiation generating structure 52 as the n-type contact layer 50.

The device structure 40A further includes a superlattice layer 12, which can be formed as described herein. Superlattice layer 12 is shown located on the same side of the radiation generating structure 52 as the p-type contact layer 54. In an embodiment, the superlattice layer 12 is at least partially transparent to radiation generated by radiation generating structure 52. It is understood that superlattice layer 12 is only illustrative of the types of superlattices that can be included in the device structure 40A. For example, the device structure 40A could include superlattice 32 and/or a variant of the superlattices shown herein.

The device structure 40A also can include an electron blocking layer 56, which can be located between the superlattice layer 12 and the radiation generating structure 52. In an embodiment, the electron blocking layer 56 has a thickness in a range between approximately two and approximately one hundred nanometers. The electron blocking layer 56 can comprise a p-type composition having a larger band gap than the barrier(s) located within the superlattice layer 12, which can result in an improved transparency of the electron blocking layer to radiation generated by the radiation generating structure 52. Furthermore, the electron blocking layer 56 can comprise a graded composition, which can be configured to decrease a resistance of the electron blocking layer 56. For example, the electron blocking layer 56 can have a graded doping that increases or decreases by approximately 10⁴ cm⁻³, e.g., between approximately 10¹⁶ and approximately 10²⁰ cm⁻³. Alternatively, the electron blocking layer 56 can have a homogeneous doping within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³.

The device structure 40A can include a contact 60. Contact 60 can comprise any type of contact. In an embodiment, the contact 60 comprises a p-type metal contact, such as a Schottky contact, a leaky Schottky contact, a rectifying contact, and/or the like. In a more specific embodiment, the contact 60 at least partially reflects the radiation generated by the radiation generating structure 52 and can be formed from, among other things, aluminum, enhanced aluminum, aluminum silicon monoxide, aluminum magnesium fluoride, rhodium, enhanced rhodium, gold, and/or the like. In another more specific embodiment, the contact 60 is at least partially transparent to the radiation generated by the radiation generating structure 52 and can be formed from, among other things, a metallic superlattice, in which each layer is at least partially transparent to the radiation. In either case, the contact 60 can be directly adjacent to a transparent adhesion layer 58. The transparent adhesion layer 58 can be configured to improve ohmic properties of the contact 60 and promote adhesion of the contact 60 to a surface of the semiconductor (e.g., layer 54). In an embodiment, the transparent adhesion layer 58 is formed of nickel. However, it is understood that transparent adhesion layer 58 can be formed of any suitable material, including Nickel oxyhydroxide (NiOx), Palladium (Pd), Molybdenum (Mo), Cobalt (Co), and/or the like.

The various layers in the device structure 40A can be formed using any type of materials. In an embodiment, the device structure 40A comprises a group III nitride-based heterostructure, in which one or more of the layers 50, 56, 12, and 54 and radiation generating structure 52 are formed of various group III nitride materials using any solution. Additionally, contact 60 can be implemented without a transparent adhesion layer 58, and can be formed of one or more layers of metal, such as for example, one or more layers of titanium, aluminum, gold, chromium, nickel, platinum, lead, rhodium, and/or the like. In an embodiment, the p-type contact layer 54 comprises a highly doped layer. In a more particular embodiment, the p-type contact layer 54 comprises a layer of gallium nitride having a doping concentration of at least 10¹⁸ dopants/cm³ and a thickness between 1-100 nanometers.

In an embodiment, one or more of the contacts 50, 54, 60 comprises graphene, which can be configured to be transparent to radiation generated by the radiation generating structure 52 and very conductive. For example, the p-type contact layer 54 to the superlattice layer 12 and/or contact 60 can be at least partially formed of p-type graphene. Similarly, the n-type contact layer 50 can be at least partially formed of n-type graphene. In an embodiment, a contact 50, 54, 60 comprises a graphene composite contact, which includes a graphene sub-layer adjacent to a thin sub-layer of metal, which can improve current spreading in the contact 50, 54, 60. In a further embodiment, the graphene composite contact is at least partially transparent to the radiation generated by the radiation generating structure 52. It is understood that the device structure 40A can include one or more layers, such as transparent adhesion layer 58 and/or contact 60, adjacent to a contact formed of graphene, such as contact 54, which are configured to improve light extraction from the device structure 40A, e.g., via a textured surface.

As shown in FIG. 6B, a light emitting device structure 40B can include a substrate 47 (e.g., sapphire and/or the like) and a buffer layer 49 (e.g., aluminum nitride and/or the like), on which the n-type contact layer 50A is formed. A metallic n-type contact 51 can be formed on a first portion of the surface of the n-type contact layer 50A and a mesa structure can be formed on a second portion of the surface of the n-type contact layer 50A. The mesa structure can include an n-type layer 50B, which can be formed on the n-type contact layer 50A through overgrowth and/or the like. The mesa structure further includes a radiation generating structure 52, a superlattice layer 12, and a p-type contact layer 54, each of which can be configured as described herein.

A metallic contact 62 can be formed over the p-type contact layer 54. As illustrated, the p-type contact 62 can include patterning. For example, the p-type contact 62 can have a mesh structure. An illustrative mesh structure can be formed from two sets of parallel strips of metal intersecting at a right angle to each other. The patterning of the p-type contact 62 can be configured to, for example, improve light extraction from the light emitting device structure 40B. For example, the patterning will reduce coverage of the p-type contact layer 54 by the p-type contact 62, which can result in reduced absorption. The patterning can comprise any configuration designed to reduce overall coverage of the p-type contact layer 54. In an embodiment, the voids within the p-type contact 62 can have a characteristic size (e.g., largest diameter) that is smaller or comparable to the current spreading length within the p-type contact layer 54.

While not shown for clarity, it is understood that a material can be located in the openings formed by the patterning of the contact 62 such that the material can contact the p-type contact layer 54. For example, the material can comprise a dielectric, such as silicon dioxide, anodized aluminum oxide, sapphire, calcium fluoride, magnesium fluoride, hafnium, and/or the like. In an embodiment, the dielectric material can be selected to be transparent to the radiation generated by the radiation generating structure 52. In a further embodiment, such a transparent dielectric material can be covered by a reflective metallic layer.

Additionally, the device structure 40B can include a capping layer 64 formed on the contact 62. While shown as being formed over only a portion of the contact 62 to illustrate the structure of the contact 62, it is understood that the capping layer 64 can cover all of the contact 62. The capping layer 64 can be formed of a material capable of exerting stress on the semiconductor structure, which can result in modification to the polarization and/or electronic properties of the device structure 40B. Illustrative materials for the capping layer 64 include semiconductors or insulators, such as MgO, ZnO, AlZnO, SiO₂, AAO, diamond film, and the like. Alternatively, the capping layer 64 can comprise one or more layers of material reflective of the radiation generated by the radiation generating structure 52. For example, the capping layer 64 can comprise: a layer of aluminum, rhodium, and/or the like; a multi-layered metallic film with layers that can be nickel, rhodium, chromium, aluminum, palladium, and/or the like; etc.

In an embodiment, a period of a structure described herein, and/or one or more sub-layers of such a period, can have a composition that varies laterally and/or in the growth direction. For example, FIGS. 7A and 7B show side and facing views, respectively, of an illustrative structure 72 according to an embodiment. In an embodiment, the structure 72 can comprise a p-type superlattice, which can be fabricated and incorporated into a device structure as shown and described in U.S. Pat. No. 9,385,271, which is hereby incorporated by reference. In a more particular illustrative embodiment, a composition of the barriers and quantum wells in the superlattice can comprise Al_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N, where molar fractions x and y correspond to the molar fractions of barriers and wells, respectively, and where the molar fraction x is greater than the molar fraction y. However, it is understood that other group III nitride materials can be utilized, such as an Al_(x)B_(y)In_(z)Ga_(1-x-z)N group III nitride material, with 0≦x, y, z≦1, and x+y+z≦1.

As illustrated in FIG. 7B, one or more of the periods 74 of the structure 72 can have a laterally non-uniform composition. For example, the period 74 is shown having laterally varying molar fractions of aluminum, which creates regions with high aluminum molar fractions and regions with low aluminum molar fractions. It is understood that aluminum is only illustrative of a group III element, the composition of which can vary laterally and the molar fraction for any combination of one or more group III elements present in the material can vary laterally. As also illustrated in FIG. 7B, one or more of the periods 74 can have a laterally non-uniform doping profile.

The non-uniformity of the composition and/or doping profile of a period 74 exceeds the non-uniformity inherent to the limitations of a growth process. In an embodiment, the compositional non-uniformity varies by at least one percent. In an embodiment, the period 74 can have a doping level of at least approximately 1×10¹⁷ dopants/cm³, while the doping non-uniformity can vary by at least fifty percent. In an embodiment, a size of doping and/or compositional non-uniformities can be several nanometers (e.g., three nanometers) or larger. The creation of such non-uniformities and the scale of such non-uniformities can be controlled, for example, by adjusting one or more growth conditions, such as growth temperature, ratio of the precursors, the V/III ratio of each precursor, and/or the like, to purposefully induce the creation of laterally non-uniform regions.

In an embodiment the period 74 is grown to have a characteristic length scale of compositional variations smaller than a distance between dislocation cores. The characteristic length scale of compositional variations can be defined, for example, as the square root of the area of compositional variations, where a compositional variation is defined as a semiconductor region with increased or decreased composition relative to an average composition of the layer by at least a target amount. For example, a compositional variation can be defined as a difference in composition that results in a bandgap of the material that differs from the bandgap of the average composition by at least thermal energy (kT). Furthermore, an average distance between compositional inhomogeneous regions can be less than an ambipolar diffusion length. For example, one of the periods 74 within a superlattice can have a characteristic size of plurality of compositional inhomogeneities that is smaller than the inverse of a dislocation density for the period, which allows for a decrease in carrier trapping by dislocations resulting in increase of hole injection into the active region of the device. A characteristic size of the varying dopant regions can be substantially the same (e.g., within +/−ten percent) as the characteristic size of the compositional inhomogeneous regions.

The lateral variation can be configured to create regions of high transparency and/or conductivity. For example, an embodiment of a period 74 can include a set of high transparency regions, each of which has a transmission coefficient for a target wavelength greater than or equal to approximately sixty percent. The set of high transparency regions can be present in at least ten percent of a lateral cross section area of the period 74. Additionally, a period 74 can include a set of high conductivity regions. The high conductivity regions can occupy a sufficient amount of the lateral cross section area of the period 74 to have an average resistance per unit area to a vertical current flow resulting in a total voltage drop across the superlattice layer of less than ten percent of a total voltage drop across the structure.

A structure described herein can include one or more additional features to, for example, control stress and/or polarization within the structure. To this extent, FIGS. 8A and 8B show band variations of an illustrative structure (e.g., a p-type superlattice) according to embodiments. While not shown for clarity, it is understood that the band variations can include a more shallow profile as shown and described herein, e.g., in conjunction with FIG. 3. In FIG. 8A, barrier layers of a structure described herein can include modulation doping in the vicinity of the quantum well layer as indicated by the domains 76. As shown in FIG. 8B, a period can include one or more interlayers, each of which comprises a thin layer of a semiconductor material with a composition that can be substantially different from the composition of the period. In an embodiment, the interlayer has a thickness of at most fifty percent of the smallest sub-layer. In a more particular embodiment, a thickness of the interlayer is between 1-20 atomic layers.

The composition can comprise a group III nitride composition, where the molar fraction of at least one group III element in the composition differs from the composition(s) of the period by at least five percent. In an embodiment, an interlayer is located in a period including two sub-layers having compositions Al_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N, respectively, and the interlayer has a composition of Al_(z)Ga_(1-z)N. In a more particular embodiment, the molar fractions are in ranges of: 0<x<0.5; 0.3<y<1; and 0.8<z<1. In another embodiment, the molar fraction for the interlayer is in a range of 0<z<0.2.

As illustrated, a barrier layer can include one or more interlayers 78A, 78B, and a quantum well can include one or more interlayers 80. Insertion of such interlayers can affect an overall stress within the period and can induce additional polarization field within the period. A thickness and composition of each interlayer 78A, 78B, 80 can be chosen not to impede overall conductivity of the structure. In an embodiment, for periods corresponding to barrier layers being under tensile stress due to presence of quantum well layers with higher gallium nitride content, the barriers can have one or more aluminum nitride interlayers 78A, 78B that induce some additional compression within the barrier layers. Furthermore, the quantum wells can contain one or more gallium nitride interlayers 80 to reduce some compression within the quantum wells. It is understood that these are examples, and other single or few atomic plane interlayers of various compositions are possible.

FIG. 9 shows band variation and doping of an illustrative structure (e.g., a p-type superlattice) according to an embodiment. While not shown for clarity, it is understood that the band variations can include a more shallow profile as shown and described herein, e.g., in conjunction with FIG. 3. In this case, the composition of the periods of the structure (e.g., barriers and quantum wells) can be selected such that the dopant ionization energy levels 82 closely match (i.e., are within 1 or 3 thermal energies, kT) one of: a valence energy band edge of the quantum well or a ground state level of holes 84 in the quantum well at the high valence band interface 91B of the quantum well and barrier as illustrated. In an embodiment, the dopant concentration across a barrier can be variable as illustrated by the dopant distribution 86. For example, a distribution of dopants is shown as being higher at the high valence band interface 91B between the barrier 90 and quantum well 92. In an embodiment, the dopant concentration at the barrier's high valence band interface 91B (or in close proximity to such interface) can be 50% or more higher than the dopant concentration at the barrier's low valence band interface 91A. As used herein, close proximity means within the range of 0-1 nm.

In an embodiment, a structure described herein can include one or more layers having a composition selected such that the layer has a transparency of at least a target transparency to radiation, such as ultraviolet radiation, of a target set of wavelengths. The layer can comprise, for example, a p-type contact layer 54 (FIG. 6), an electron blocking layer 56 (FIG. 6), a superlattice layer 12 (FIG. 6), and/or the like. For example, a layer can be a group III nitride-based layer, which is composed of Al_(x)Ga_(1-x)N where the aluminum molar fraction (x) is sufficiently high in some domains of the layer to result in the layer being at least partially transparent to ultraviolet radiation. In an embodiment, the layer can comprise a superlattice layer located in an emitting device configured to emit radiation having a dominant wavelength in the ultraviolet spectrum, and the composition of at least one sub-layer in each period of the superlattice layer is configured to be at least partially transparent to ultraviolet radiation having a target wavelength corresponding to the ultraviolet radiation emitted by the emitting device.

In an embodiment, the sub-layer has a thickness in a range between approximately one and approximately one thousand nanometers. Furthermore, the sub-layer can have a graded doping that increases or decreases by approximately 10⁴ cm⁻³, e.g., between approximately 10¹⁶ and approximately 10²⁰ cm⁻³. Alternatively, the sub-layer can have a homogeneous doping within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³. The doping can be any type of doping. For example, the doping can be: modulation doping; unintentional doping by impurities from one or more of oxygen, hydrogen, and magnesium; a dopant, such as magnesium and/or carbon, diffused from another doped layer or present in the growth chamber as residual elements; and/or the like. In an embodiment, one or more sub-layers can be co-doped with magnesium and carbon, where both the carbon and magnesium doping levels are within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³, but the combined concentration of the dopants does not exceed approximately 10²⁰ cm⁻³. In another embodiment, the doping can alternate between two or more dopants. For example, a sub-layer can include carbon doping, while the adjacent sub-layer(s) can include magnesium doping.

An amount of transparency of a short period superlattice (SPSL) can be approximated by computing the averaged band gap of the SPSL, and deducing average absorption coefficient of the SPSL. The absorption coefficients depend on an absorption edge of the semiconductor material, which for materials formed of an AlGaN alloy, is a function of the molar fractions of the Al_(x)Ga_(1-x)N semiconductor alloy.

In an embodiment, the target transparency for the material is at least ten times more transparent than the least transparent layer of material in the structure (e.g., GaN for a group III nitride-based device). In this case, an absorption coefficient of the semiconductor layer can be on the order of 10⁴ inverse centimeters or lower. In this case, a one micron thick semiconductor layer will allow approximately thirty-six percent of the ultraviolet radiation to pass there through.

FIG. 10 shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al_(x)Ga_(1-x)N alloy according to an embodiment. In order to maintain an absorption coefficient of the semiconductor layer at orders of 10⁴ inverse centimeters or lower, the content of aluminum in an SPSL barrier layer can be chosen based on the corresponding target wavelength or range of wavelengths. For example, for a target wavelength of approximately 250 nanometers, the aluminum molar fraction can be approximately 0.7 or higher, whereas for a target wavelength of approximately 300 nanometers, the aluminum molar fraction can be as low as approximately 0.4. FIG. 11 shows an illustrative chart for selecting an aluminum content of an Al_(x)Ga_(1-x)N alloy to maintain a target transparency for a corresponding emitted wavelength, λ, according to an embodiment. In this case, the target transparency corresponds to an absorption coefficient of the semiconductor layer on the order of 10⁴ inverse centimeters. Note that in FIG. 11, the dependence of x=x(A) is linear, with x=C·λ+B, where C=−0.0048 nm⁻¹, and B=1.83.

In an embodiment, one or more sub-layers of the SPSL can have a graded composition. For example, a sub-layer of the SPSL can be formed of an Al_(x)Ga_(1-x)N alloy, where the aluminum molar fraction, x, is continually varied in the vertical direction of the sub-layer.

In an embodiment, a device can include one or more layers with lateral regions configured to facilitate the transmission of radiation through the layer and lateral regions configured to facilitate current flow through the layer. For example, the layer can be a short period superlattice, which includes barriers alternating with wells. In this case, the barriers can include both transparent regions, which are configured to reduce an amount of radiation that is absorbed in the layer, and higher conductive regions, which are configured to keep the voltage drop across the layer within a desired range. As used herein, the term lateral means the plane of the layer that is substantially parallel with the surface of the layer adjacent to another layer of the device. As described herein, the lateral cross section of the layer can include a set of transparent regions, which correspond to those regions having a relatively high aluminum content, and a set of higher conductive regions, which correspond to those regions having a relatively low aluminum content.

The set of transparent regions can be configured to allow a significant amount of the radiation to pass through the layer, while the set of higher conductive regions can be configured to keep the voltage drop across the layer within a desired range (e.g., less than ten percent of a total voltage drop across the structure). In an embodiment, the set of transparent regions occupy at least ten percent of the lateral area of the layer, while the set of higher conductive regions occupy at least approximately two percent (five percent in a more specific embodiment) of the lateral area of the layer. Furthermore, in an embodiment, a band gap of the higher conductive regions is at least five percent smaller than the band gap of the transparent regions. In a more particular embodiment, the transparent regions comprise a transmission coefficient for radiation of a target wavelength higher than approximately sixty percent (eighty percent in a still more particular embodiment), while the higher conductive regions have a resistance per unit area to vertical current flow that is smaller than approximately 10⁻² ohm·cm². As used herein, the term transmission coefficient means the ratio of an amount of radiation exiting the region to an amount of radiation entering the region.

The transparent and conductive regions can be formed using any solution. For example, a layer can be grown using migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). During the growth, inhomogeneities in the lateral direction of a molar fraction of one or more elements, such as aluminum, gallium, indium, boron, and/or the like, can be allowed in the layer. In an embodiment, such compositional inhomogeneities can vary by at least one percent.

In an embodiment, a light emitting device structure can include one or more structures configured to reduce an overall polarity of the structure. In embodiment, the structure can form a cladding layer, a p-type contact layer, and/or the like, of a light emitting device. In order to confine a polarization charge within a sub-layer, the sub-layer thicknesses can be larger than a Bohr radius of the carriers. Using a p-type contact layer as an illustrative example, the Bohr radius, R_(B), can be calculated for hole carriers. In this case, the Bohr radius is given by R_(B)=4π∈h²/m_(h)e², where ∈ is the permittivity of the material, h is the reduced Planck's constant, m_(h) is the hole rest mass, and e is the elementary charge. For the composition Al_(0.5)Ga_(0.5)N, the mass of an “average” hole is about four times the electron rest mass (m_(h)˜4m_(e)), the permittivity is approximately nine times the permittivity of free space (∈˜9∈₀), and the resulting Bohr radius, R_(B), is approximately 9/4 of the Bohr radius of hydrogen, R_(H), that is R_(B) 1.2 nm. A group III semiconductor layer having a higher concentration of gallium will have a smaller hole mass (e.g., for GaN, m_(h)˜1.4). As a result, such a group III semiconductor layer can have a Bohr radius, R_(B)˜6×R_(H)=3.2 nm.

An AlGaN film deposited by MOCVD on a substrate formed of sapphire, SiC, Si, and/or the like, typically grows with its gallium face up. This growth corresponds to the growth direction of the film being [0001], the positive c-axis direction. However, growth of a heavily Mg-doped AlGaN layer by MOCVD can produce a negative c-axis direction (N-face growth) of AlGaN. The inversion of polarity can reduce an overall “average” polarity within a given sub-layer. To this extent, FIG. 12 shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment. As illustrated the layer includes a plurality of lateral domains, at least one of which is a nitride facing domain (N-face) and at least one of which is a gallium facing domain (Ga-face). As illustrated, the polarization (Ps) and electric field (E) vectors are inversed on either side of the boundary between the domains.

In an embodiment, polarization control of a group III nitride semiconductor layer, such as a sub-layer in a superlattice layer described herein, is accomplished using an alloy, such as Al_(x)In_(y)B_(z)Ga_(1-x-y-z)N, where 0≦x, y, z≦1 and 0≦x+y+z≦1. A group III nitride semiconductor layer deposited on another group III nitride semiconductor layer can exhibit both piezoelectric and spontaneous polarization. These polarizations can compensate one another, resulting in a lowering of a polarization of the deposited semiconductor layer.

For example, FIG. 13 shows possible molar fractions of indium and aluminum in an AlInGaN layer to achieve zero total polarization for the AlInGaN layer when grown over an AlN layer according to an embodiment. As illustrated, as an aluminum molar fraction increases in the AlInGaN alloy, a molar fraction for indium also needs to increase to achieve zero total polarization for the AlInGaN layer. In an embodiment, any combination of Al and In molar fractions, which result in a zero total polarization for the AlInGaN layer is selected. In a more particular embodiment, an Al_(0.4)In_(0.2)Ga_(0.4)N layer can be used to achieve zero total polarization for the AlInGaN layer.

In an embodiment, an alloy can be selected to provide a target polarization for the corresponding layer. The target polarization can be positive or negative, any magnitude within a range of polarization magnitudes, and/or the like. Continuing to use an AlInGaN layer grown on an AlN layer as an illustrative example, FIG. 14 shows contour lines of polarization for various combinations of molar fractions of indium and aluminum in an AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment. As noted, the thick line corresponds to a substantially zero total polarization for the AlInGaN layer as shown in FIG. 13. For molar fractions of indium and aluminum having a higher relative indium content than that providing a zero total polarization (e.g., contour lines located above the zero total polarization line), a positive net polarization is present within the AlInGaN layer, which can result from piezoelectric polarization dominating over spontaneous polarization in the AlInGaN material. Conversely, for molar fractions of indium and aluminum having a higher relative aluminum content than that providing zero total polarization (e.g., contour lines located below the zero total polarization line), a negative net polarization is present within the AlInGaN layer, which can result from spontaneous polarization dominating over piezoelectric polarization in the AlInGaN material.

As illustrated by FIGS. 13 and 14, having a quaternary group III nitride semiconductor alloy, such as AlInGaN, allows for a thorough control of both the sign and magnitude of the net polarization in the semiconductor material. However, it is understood that FIGS. 13 and 14 are only illustrative of various embodiments, which can be used to control and affect polarization within a semiconductor layer of any type. For example, the addition of boron to the AlInGaN material also can further influence the resulting polarization. Similarly, the base layer can comprise another material, such as another group III nitride material, other than AlN. Furthermore, selection of and deposition of neighboring semiconductor layers can impact polarization within the semiconductor layer due to the presence of lattice mismatch stresses at the layer interfaces, which can result in increased piezoelectric polarization. The polarization within the semiconductor layer can be evaluated using expressions for piezoelectric and spontaneous polarization known in art of the group III nitride semiconductors.

Structures 10, 30 described herein can be incorporated as part of, for example, a transistor (e.g., a field effect transistor), a photodetector, a monolithic and/or optoelectronic integrated circuit, a metal-semiconductor diode, a p-n junction diode, a switch, and/or the like. In this case, the device can include one or more superlattices as buffer layer(s), barrier layer(s), contact layer(s), and/or the like. In a more particular embodiment, the periods of the superlattice layer are formed from AlInN.

While shown and described herein with respect to the fabrication of a superlattice layer, it is understood that an embodiment of the invention can be applied to the fabrication of a heterostructure comprising a set of quantum wells and a set of barriers. The various sub-layers shown and described herein can be formed using any solution. For example, the superlattice layers 12, 32 can be grown using a combination of metallo organic chemical vapor deposition (MOCVD) and/or migration enhanced MOCVD (MEMOCVD), in which each period in the superlattice layer 12, 32 requires at least two growth steps.

While shown and described herein as a method of designing and/or fabricating a structure and/or a corresponding semiconductor device including the structure, it is understood that aspects of the invention further provide various alternative embodiments. For example, in one embodiment, the invention provides a method of designing and/or fabricating a circuit that includes one or more of the semiconductor devices designed and fabricated as described herein (e.g., including one or more superlattice layers 12, 32).

To this extent, FIG. 15 shows an illustrative flow diagram for fabricating a circuit 126 according to an embodiment. Initially, a user can utilize a device design system 110 to generate a device design 112 using a method described herein. The device design 112 can comprise program code, which can be used by a device fabrication system 114 to generate a set of physical devices 116 according to the features defined by the device design 112. Similarly, the device design 112 can be provided to a circuit design system 120 (e.g., as an available component for use in circuits), which a user can utilize to generate a circuit design 122 (e.g., by connecting one or more inputs and outputs to various devices included in a circuit). The circuit design 122 can comprise program code that includes a device designed using a method described herein. In any event, the circuit design 122 and/or one or more physical devices 116 can be provided to a circuit fabrication system 124, which can generate a physical circuit 126 according to the circuit design 122. The physical circuit 126 can include one or more devices 116 designed using a method described herein.

In another embodiment, the invention provides a device design system 110 for designing and/or a device fabrication system 114 for fabricating a semiconductor device 116 by using a method described herein. In this case, the system 110, 114 can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the semiconductor device 116 as described herein. Similarly, an embodiment of the invention provides a circuit design system 120 for designing and/or a circuit fabrication system 124 for fabricating a circuit 126 that includes at least one device 116 designed and/or fabricated using a method described herein. In this case, the system 120, 124 can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the circuit 126 including at least one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer program fixed in at least one computer-readable medium, which when executed, enables a computer system to implement a method of designing and/or fabricating a semiconductor device as described herein. For example, the computer program can enable the device design system 110 to generate the device design 112 as described herein. To this extent, the computer-readable medium includes program code, which implements some or all of a process described herein when executed by the computer system. It is understood that the term “computer-readable medium” comprises one or more of any type of tangible medium of expression, now known or later developed, from which a copy of the program code can be perceived, reproduced, or otherwise communicated by a computing device. For example, the computer-readable medium can comprise: one or more portable storage articles of manufacture; one or more memory/storage components of a computing device; paper; and/or the like.

In another embodiment, the invention provides a method of providing a copy of program code, which implements some or all of a process described herein when executed by a computer system. In this case, a computer system can process a copy of the program code to generate and transmit, for reception at a second, distinct location, a set of data signals that has one or more of its characteristics set and/or changed in such a manner as to encode a copy of the program code in the set of data signals. Similarly, an embodiment of the invention provides a method of acquiring a copy of program code that implements some or all of a process described herein, which includes a computer system receiving the set of data signals described herein, and translating the set of data signals into a copy of the computer program fixed in at least one computer-readable medium. In either case, the set of data signals can be transmitted/received using any type of communications link.

In still another embodiment, the invention provides a method of generating a device design system 110 for designing and/or a device fabrication system 114 for fabricating a semiconductor device as described herein. In this case, a computer system can be obtained (e.g., created, maintained, made available, etc.) and one or more components for performing a process described herein can be obtained (e.g., created, purchased, used, modified, etc.) and deployed to the computer system. To this extent, the deployment can comprise one or more of: (1) installing program code on a computing device; (2) adding one or more computing and/or I/O devices to the computer system; (3) incorporating and/or modifying the computer system to enable it to perform a process described herein; and/or the like.

The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims. 

What is claimed is:
 1. A contact structure including: a metal layer; and a superlattice layer including a plurality of quantum wells, each of the plurality of quantum wells including: a first sub-layer having a first group III nitride composition and a first built-in electric field; and a second sub-layer immediately adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second built-in electric field opposite the first built-in electric field, wherein the first and second built-in electric fields substantially cancel one another.
 2. The structure of claim 1, further comprising a group III nitride contact semiconductor layer located between the metal layer and the superlattice layer.
 3. The structure of claim 2, wherein the contact semiconductor layer comprises a doping concentration of at least 10¹⁸ dopants/cm³.
 4. The structure of claim 2, wherein the contact semiconductor layer is gallium nitride.
 5. The structure of claim 1, wherein the superlattice layer further includes a plurality of barriers alternating with the plurality of quantum wells, and wherein each of the plurality of barriers is formed of a group III nitride material.
 6. The structure of claim 5, wherein each of the plurality of barriers includes a p-type doping at least in a region adjacent to at least one immediately adjacent quantum well.
 7. The structure of claim 6, wherein a dopant ionization energy level of a p-type dopant in each of the plurality of barriers is substantially aligned with one of: a valence energy band edge of the at least one immediately adjacent quantum well or a ground state energy for free carriers in the at least one immediately adjacent quantum well.
 8. The structure of claim 1, wherein the metal layer is patterned.
 9. The structure of claim 8, wherein the pattern has a mesh structure.
 10. The structure of claim 8, wherein the pattern includes a plurality of openings in the metal layer, and wherein the structure further includes a dielectric material located within the plurality of openings.
 11. The structure of claim 1, further comprising a capping layer located directly on the metal layer.
 12. The structure of claim 1, wherein at least one of: the first sub-layer or the second sub-layer, includes a composition that varies along lateral dimensions of the at least one of: the first sub-layer or the second sub-layer.
 13. The structure of claim 12, wherein the laterally varying composition forms at least one of: a set of high transparency regions or a set of high conductivity regions.
 14. The structure of claim 1, wherein at least one of: the first sub-layer or the second sub-layer, includes a doping concentration that varies along lateral dimensions of the at least one of: the first sub-layer or the second sub-layer.
 15. A device comprising: an active region; and a p-type contact structure located on a p-type side of the active region, the p-type contact structure including: a metal layer; and a superlattice layer including a plurality of quantum wells, each of the plurality of quantum wells including: a first sub-layer having a first group III nitride composition and a first built-in electric field; and a second sub-layer immediately adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second built-in electric field opposite the first built-in electric field, wherein the first and second built-in electric fields substantially cancel one another.
 16. The device of claim 15, further comprising a gallium nitride contact semiconductor layer located between the metal layer and the superlattice layer, wherein the contact semiconductor layer comprises a doping concentration of at least 10¹⁸ dopants/cm³.
 17. The device of claim 15, wherein the superlattice layer further includes a plurality of barriers alternating with the plurality of quantum wells, wherein each of the plurality of barriers includes a p-type doping at least in a region adjacent to at least one immediately adjacent quantum well, and wherein a dopant ionization energy level of a p-type dopant in each of the plurality of barriers is substantially aligned with one of: a valence energy band edge of the at least one immediately adjacent quantum well or a ground state energy for free carriers in the at least one immediately adjacent quantum well.
 18. The device of claim 15, wherein the metal layer has a mesh structure including a plurality of openings in the metal layer.
 19. The device of claim 15, wherein at least one of: the first sub-layer or the second sub-layer, includes a composition that varies along lateral dimensions of the at least one of: the first sub-layer or the second sub-layer.
 20. A device comprising: an active region; and a p-type contact structure located on a p-type side of the active region, the p-type contact structure including: a metal layer; and a superlattice layer including a plurality of quantum wells, each of the plurality of quantum wells including: a first sub-layer having a first group III nitride composition and a first built-in electric field; and a second sub-layer immediately adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second built-in electric field opposite the first built-in electric field, wherein the first and second built-in electric fields substantially cancel one another, wherein at least one of: the first sub-layer or the second sub-layer, includes a composition that varies along lateral dimensions of the at least one of: the first sub-layer or the second sub-layer, and wherein the laterally varying composition forms at least one of: a set of high transparency regions or a set of high conductivity regions. 